Seachem Flourish contains just 0.14% calcium so it might not be enough for the snails. I got my bottle out of the freezer to check*.
But your plants would benefit from some fertliser. Your very soft water contains very few minerals so the plants quickly use up the few there are.
If they are planted in the substrate, root tabs would be the best way to go, but avoid the API ones as they have a reputation for disintegrating and messing up the water. But with plants that are attached to decor, they need liquid fertliser and Seachem Flourish contains almost all the trace elements plants need. It doesn't contain the macro elements nitrate, potassium and phosphate (the same NPK that gardeners talk about) as nitrate is made by the fish and the other two are in fish food.
With most fertilisers, the dose rate is for tanks stuffed full of plants, so reduce the amount you use according to how many plants you have. For example, I add half the recommended dose once every two weeks.
*
One tip I have picked up - liquid fertilisers go off after they are opened, and if you don't use much the bottle lasts ages. So the best thing to do is freeze it. I gave the bottle a good shake then poured it into a few small containers, then put all but the one I'm using in the freezer (including a bit in the bottom of the bottle it came in). The current pot is in the fridge.